Conventionally, in a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, for example, a positive electrode mixture paste obtained by kneading electrode materials such as a positive electrode active material, a conductive material and a binding material (binder) together with a solvent is applied to a current collector and dried to form a positive electrode mixture layer, thereby preparing a positive electrode. In addition, a negative electrode mixture paste obtained by kneading electrode materials such as a negative electrode active material, a thickener and a binding material is applied to a current collector and dried to form a negative electrode mixture layer, thereby preparing a negative electrode.
The positive electrode, the negative electrode and a separator interposed between the positive electrode and the negative electrode are wound to form an electrode body, and the electrode body impregnated with an electrolyte solution is housed in a case to form a nonaqueous electrolyte secondary battery.
The case is provided with a positive electrode terminal and a negative electrode terminal each protruding to the outside, and the positive electrode terminal and the negative electrode terminal are connected to the positive electrode and the negative electrode through a first collecting terminal and a second collecting terminal, respectively.
A current interrupt device is interposed between the positive electrode terminal and the first collecting terminal. If the pressure inside the case becomes higher than a predetermined value, the current interrupt device operates to interrupt electrical connection between the positive electrode terminal and the first collecting terminal, which is a current path of the nonaqueous electrolyte secondary battery.
The electrolyte solution in the nonaqueous electrolyte secondary battery contains an additive that is decomposed on the surface of the positive electrode to generate gas at the time of overcharge of the nonaqueous electrolyte secondary battery, and the gas generated by the additive increases the pressure inside the case.
For properly operating the current interrupt device, it is important to efficiently generate gas at the time of overcharge of the nonaqueous electrolyte secondary battery. For efficiently generating gas and efficiently releasing the generated gas from the positive electrode mixture layer, it is effective to make the positive electrode mixture layer of the positive electrode porous.
The size of voids (pores) between positive electrode active material particles can be increased to improve the porosity of the positive electrode mixture layer when the particle diameter of the positive electrode active material contained in the positive electrode mixture layer is made large (e.g., 7 to 10 μm) as shown in FIG. 4, for example. However, when the particle diameter of positive electrode active material particles is increased as described above, electric conductivity in particles and between particles becomes insufficient. As a result, the internal resistance of the nonaqueous electrolyte secondary battery increases, leading to deterioration of the high-rate characteristics, for example, at the time of discharging the battery at a large current.
As described in Patent Literature 1, when the positive electrode mixture layer is formed, microbubbles are added to a positive electrode mixture paste to reduce the density of the positive electrode mixture paste, and the positive electrode mixture paste is applied to a current collector to prepare the positive electrode, which may improve the porosity of the positive electrode mixture layer.
However, the technique described in Patent Literature 1 has the following problem: microbubbles are added throughout the positive electrode mixture paste, and many voids exist not only on a surface of the positive electrode mixture layer but also in the vicinity of the current collector. Therefore, electric conductivity in particles and between particles of the positive electrode active material becomes insufficient, and the internal resistance of the positive electrode increases, leading to deterioration of the high-rate characteristics of the nonaqueous electrolyte secondary battery.
On the other hand, as shown in FIG. 5, if the particle diameter of positive electrode active material particles is made small (e.g., 2 to less than 7 μm) as a whole, conductivity in particles and between particles can be secured, and high-rate characteristics can be improved. However, the distance between particles decreases, and the size of voids between positive electrode active material particles is reduced. As a result, generation of gas in the positive electrode mixture layer is suppressed at the time of overcharge, and the generated gas is hard to be released from the inside of the positive electrode mixture layer, which makes it difficult to reliably operate the current interrupt device.